In a phased array system such as that used for ultrasonically performing diagnostic procedures, the paths from a point being scanned to the various transducers of the scan aperture are of different lengths, resulting in echoes from the point being received at the transducers at different times. In conventional systems of this type, such as the HP SONOS 1000 ultrasound system from Hewlett-Packard, analog techniques, such as mixers and delay lines, are utilized to compensate for these different path lengths, so that when the outputs from the various transducers are summed, the summed values are for the same scan point. This process is referred to as beam formation.
However, once the beam is formed, the information represents analog data along either straight or radial lines. The samples along these lines bear no relationship to the locations of pixels on a cathode ray tube screen where the scanned image is normally to be displayed. Thus, the analog beam formed data is typically sampled and applied to a digital scan converter to map this information into a frame buffer for controlling image display. Systems for performing such scan conversion operations are described, for example, in U.S. Pat. Nos. 4,468,747 and 4,471,449, both filed in the name of Steven C. Leavitt et al and assigned to the assignee of the instant application.
However, while such systems provide good results, there are a number of reasons why a fully digital front end is preferable to the partially analog and partially digital systems currently employed. First, the elimination of costly analog delay elements reduces the overall cost of the system. Second, a fully digital system is more flexible and thus adapted to provide higher degrees of accuracy. For example, the compensation required for beam formations varies as a function of scan depth. With tapped delay lines, it is difficult to achieve required delay variations. Many systems merely switch delays at some depth or depths, resulting in discontinuities in the output. More sophisticated systems can still only approximate continuous delay variations. A digital front end would provide the flexibility required to provide substantially continuous depth compensation in beam formation. Third, a fully digital system is less prone to error. A digital system may also process multiple received lines from a single transmit, something which is not possible without using two or more costly analog delay lines.
Further, there are currently some redundancies in the operation which results in analog beam formation and in the operation for doing digital scan conversion. Thus, with a fully digital front end, both complexity and cost can be further reduced by combining these operations into a single operation which compensates for the difference in scan line lengths for beam formation at the same time that the scanned image is being converted to provide image values at the pixel locations to control the display.